A matching circuit is understood in electrical engineering as a circuit arrangement which allows the matching of an electrical signal value, for example, a current, a voltage and/or a power, to given conditions, for example, to an input condition, an output condition and/or a transmission condition within circuits and/or between circuit units. Given conditions are especially an impedance value, a resistance value, and inductance value and/or a capacitance value of an electrical circuit unit.
In particular, different circuit units are coupled with one another through matching circuits, wherein the transmission of the electrical signal value is significantly influenced through the use of the matching circuit. The relationship of an output condition of an energy-supplying circuit unit to the input condition of a circuit unit downstream of the latter is decisive for the embodiment of the matching circuit.
If the output condition, for example, the output impedance of the energy-supplying circuit unit, and the input condition, for example, the input impedance of the subsequent circuit unit, are of identical magnitude, reflection matching and power matching is present. In this case, the transmission pathway comprising, for example, an energy-supply circuit unit and a subsequent circuit unit, is homogenous and no reflection of the electrical signal value, especially the blind component of the signal value, occurs at the output respectively the input of the circuit units. Especially in high-frequency technology and in communications technology, it is often required that circuit units must be matched in a reflection-free manner, wherein a power matching is also required, at least indirectly.
Electrical signals with very high frequencies are regarded as electromagnetic waves. Such signals are transmitted from point to point via transmission lines, wherein the lines are characterized by their length, their propagation constant β and their line impedance Zc. The line impedance of a transmission line is regarded as a pure ohmic resistance if no substantial losses are present. Ideally, the endpoint of a transmission line is terminated with an ohmic resistance element, wherein the resistance value is identical to the ohmic resistance value of the characteristic impedance Zc of the line. In the case of non-ideal matching of the output of the transmission line, a part of the electrical signal is reflected back from the output of the line to the input of the line. Such reflections generate ripples in the magnitude of the frequency response of the overall transmission path, since the magnitude of the frequency response is dependent upon the line conditions at both ends of the transmission line.
For measuring instruments, for example, oscilloscopes or spectrum analyzers, which are supposed to register high-frequency signal values, it is important that a high precision input impedance is present at the input of the measuring instrument, wherein the input impedance must be independent of the input signal. Reflections at the input of the measuring instrument must absolutely be prevented to avoid causing a distortion of the electrical signal. The input impedance for signal values with frequencies from DC up to very high frequencies must therefore be a purely resistive impedance in order to prevent such ripples in the magnitude of the frequency response.
Such ripples of the magnitude lead to undesired frequency-dependent measurement errors which can seriously falsify the amplitudes of the input signal to be registered.
Such ripples of the magnitude cannot simply be calibrated out, because, for example, the line length and the propagation constant β of the line, as substantial factors for the fluctuations, are located outside the control of a measuring instrument.
FIG. 1a and FIG. 1b illustrate the input of a measuring instrument with an exemplary high-frequency transistor in simplified form. Typically, a series resistance element RT connected to a virtual ground point is used for inputs to a measuring instrument, see FIG. 1a or a parallel resistance element RT is used at the input, see FIG. 1b. 
Even if the ohmic resistance element RT is embodied very precisely, the input impedance varies with frequency of the electrical input signal because of parasitic inductances or parasitic capacitances of the transistor or of the resistance element RT itself. The connecting elements between resistor and transistor also trigger such parasitic effects.
The effect of parasitic units based on the connecting elements is particularly problematic when the input transistor is embodied on a semiconductor substrate, while the resistance element RT is to be arranged outside this semiconductor substrate, for example, because of space requirements, interference coupling requirements or heat removal specifications. Even if the resistance element RT were to be embodied on the semiconductor substrate, the considerable tolerances from 10 to 15% which might be expected with resistance elements embodied in this manner would make a use in the field of measurement technology impossible. Furthermore, such resistance elements are susceptible to electrostatic discharge processes and/or current/voltage peaks of the input signal. Furthermore, such resistance elements are strongly temperature dependent. Furthermore, such resistance elements form a large parasitic capacitance on the semiconductor substrate. For these reasons, such resistance elements cannot be used as a terminating element respectively matching element for a high-precision measuring instrument in high-frequency technology.
Consequently, it is not possible to dispense with the use of non-ideal resistance elements as matching elements. It is therefore desirable to optimize a non-ideal input resistance of a measuring instrument by means of a matching circuit in such a manner that a purely ohmic effect with a very constant resistance value is obtained for all input frequencies.
A great many matching circuits which can be used for various circuit units are known from the prior art. In this context, a merely narrow-band matching circuit, which achieves a matching of the matching element only in the immediate proximity of the operating frequency of the high-frequency circuit, is conventional for high-frequency applications. Such a matching circuit can be achieved for an arbitrary impedance value, for example, through a global arrangement of inductances or capacitances as a compensation element or through an additional stub in the transmission line. Such a matching circuit is calculated in an ideal manner for a single frequency. A deviation from this frequency during operation automatically leads to an error-matched circuit unit because of the deviating impedance values occurring.
A different approach to the solution for the above named problem is the use of a so-called tapered transmission line. In microstrip lines, the characteristic impedance is dependent upon the width of the transmission lines. If a source with a given impedance, for example 75 ohms, must be matched with another impedance value, for example, 50 ohms, a microstrip line of which the width tapers stepwise from a first width to a second width can be used. The first width causes an impedance value of 75 ohms, whereas the second width causes an impedance value of 50 ohms. In the case of high-frequency alternating signals, a better matching is achieved in this manner than, for example, merely a change in width. These tapered transmission lines can accordingly be controlled in an adequate, if not actually perfect manner for every required bandwidth, in order to achieve an ideal matching for the given conditions. In this context, the operating frequency range in which the transmission line is to be matched as ideally as possible, is substantially enlarged. However, good impedance matching in the lower frequency range and especially for direct signals is not possible with such matching circuits.
All of the methods previously described for the solution of this problem can be used only for lossless matching circuits subject to the specification that a power transmission is to be maximized in order to transmit a signal from an energy-supply circuit unit to a downstream circuit unit.
By contrast, if the purpose of a matching circuit is to eliminate ripples due to reflections at the output of a transmission line caused by impedance mismatch, it is acceptable that a part of the energy does not arrive in the receiver.
One solution for the problem named above is the use of a broadband attenuation element in series configurations to the input of the circuit unit, for example, a measuring instrument. In this context, the incoming wave of the input signal is attenuated by this attenuator. The reflected wave resulting from the error matching is attenuated again on the return path by the same attenuator. The incoming wave and the reflected wave are superposed. As a result, the reflected wave obtained is reduced by twice the attenuation factor of the attenuator on a logarithmic scale in decibels. The disadvantage of this solution is that the input signal is attenuated, which leads to a deterioration of the signal-noise ratio.